Scattered radiation presents a particular challenge for radiographic imaging. In some cases, scatter can significantly reduce subject contrast, making it difficult to discern various anatomical features in the radiographic image.
Linear grids have been devised in order to help correct this problem. Linear grids are antiscatter devices that are used to improve contrast and to improve the signal to noise ratio in radiographic images. A conventional antiscatter grid typically consists of a series of lead foil strips separated by spacers that are transmissive to x-rays. The spacing of the strips determines the grid frequency, and the height-to-interspacing distance between lead strips determines the grid aspect ratio. Grids can be oriented horizontally or vertically relative to the imaging medium.
There are two general types of antiscatter grids: moving (Bucky-Potter configuration) and stationary. For moving type grids, the shadows of the lead strips are blurred out by motion, which can be either reciprocating or unidirectional (single stroke). For stationary grids, the shadows of the lead strips are imposed onto the radiographic image and can be reduced using programmed image processing methods.
In general, antiscatter grids, equivalently termed “image contrast antiscatter grids”, are required for most types of “thick” tissue medical imaging procedures; i.e., procedures in which the screen is not located close (within about the thickness of the screen) to body tissue during medical imaging procedures.
Image contrast antiscatter grids have been formed in a number of ways. Grids can be formed by laminating together foils of x-ray transparent material, such as aluminum and x-ray absorbing material, such as lead, to form an extended sandwich structure. The simplified schematic of FIG. 1 illustrates a known sandwich structure image contrast antiscatter grid 24 including aluminum foils 26 and lead foils 28 forming an alternating, parallel arrangement.
Other methods of forming image contrast antiscatter grids have been described, for example, in U.S. Pat. Nos. 5,581,592 and 5,557,650, which are incorporated herein by reference in their entirety.
The various methods proposed for forming antiscatter grids have proved to be cumbersome or unsatisfactory for a number of reasons, including:                (i) high cost, due in large part to complex and expensive fabrication;        (ii) significant weight, making the grid difficult to position;        (iii) grid visibility in the acquired image; [Known image contrast antiscatter grids, such as the image contrast antiscatter grid 24 in FIG. 1, have a relatively coarse structure that produces grid lines in radiographic images. To reduce this problem, for example, the grids can be moved slightly back and forth in a direction approximately perpendicular to the normal (that is, slightly back and forth perpendicular to the direction of the x-rays) to blur the image of the grid lines formed on the receiver. This movement of the grids is known as the “Bucky system.” However, the Bucky system requires the imaging system to include additional components and, thus, increases the cost and complexity of the system.]        (iv) incomplete scatter compensation; [Known image contrast antiscatter grids, such as the image contrast antiscatter grid 24 of FIGS. 1 and 2, only remove the Compton-scattered, non-normal (off-z-axis) photons in one dimension (i.e., along either the x-axis or the y-axis). In order to provide two-dimensional photon removal using these grids, two grids, such as two of the image contrast antiscatter grids 24, must be stacked with their respective foils oriented orthogonally with respect to each other. Although the combined use of two grids may improve Compton-scattered photon removal in a second direction, the cost of the imaging system is significantly increased by the added cost of the second grid. Thus, the value of improving the performance of the imaging system by using two image contrast antiscatter grids may not justify the associated added cost and space requirements to achieve the improved performance.]        (v) fragile structure, readily damaged by mishandling;        (vi) rigidity, making the grid unusable in some applications; [Flexibility of the grid can have value in particular imaging applications, but is not currently available.] and        (vii) not readily adaptable to different imaging conditions.        
In addition, grid use increases the required patient exposure, because of the needed compensation for absorption of primary radiation by the interspace material that forms the grid.
A greatly enlarged cross sectional portion of a simple, conventional image contrast anti-scatter grid 24 is schematically shown in FIG. 2. In the grid, slats of x-ray opaque lead foil 28 alternate with filler strips of x-ray transmissive aluminum foil 26 or fiber. The height of the grid is h, and the interspace width is w. The ratio r=h/w is known as the grid ratio. In practice, for this ratio, h/w=16/1 may be considered a maximum. To achieve this ratio without reducing a transmission magnitude of the grid requires a large number of slats (i.e., a small value of w), since the available h is limited by the current use and design of x-ray equipment to values of about two millimeters. Slats, generally formed of lead, add significantly to grid weight.
Another type of grid, shown in U.S. Pat. No. 2,605,427 issued Jul. 29, 1952, to Delhumeau is a two-dimensional focusing grid, so called because the slats are aligned with the rays coming from the x-ray source. Two-dimensional anti-scatter grids can be nearly twice as heavy as one-dimensional grids due to the additional amounts of x-ray absorbent material that are needed.
U.S. Pat. No. 6,408,054 to Rahn et al. describes a micromachined contrast grid having numerous tiny holes formed by etching and photolithography, for example. The holes can be of selected depths and angles. The holes are then filled with small amounts of lead or other x-ray absorbing material to form a grid pattern. Various coating processes are described for filling the cylindrical holes formed in the grid substrate.
Among solutions proposed for grid fabrication is forming multiple sheets and aligning the sheets to each other to define the path of incident radiation through the grid. U.S. Pat. No. 4,951,305 to Moore et al. describes one approach to this problem using aligned sheets. Accurate alignment of multiple sheets to each other, however, proves to be difficult, even where extremely tight manufacturing tolerances are maintained. Moreover, subsequent handling of the grid can easily cause inadvertent misalignment of the successive sheets used in such an arrangement.
Rigid grids do not adapt to nonplanar detector surfaces and allow only a very small focus range. Because grids have traditionally been formed using lead strips separated by aluminum spacing material, grids have been formed as rigid, planar devices that do not flex. However, there may be some applications for which some amount of grid flexure is of value. Existing approaches used for grid fabrication do not allow flexibility of the grid.
It can thus be appreciated that there are advantages to grid design that helps to remedy one or more of the identified factors that make conventional grids cumbersome or unsatisfactory for use.